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Tue, 23 Jul 2019 09:41:06 +0000 (UTC) Received: from dresden.str.redhat.com (unknown [10.40.205.118]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 220155D9C5; Tue, 23 Jul 2019 09:41:04 +0000 (UTC) To: Kevin Wolf References: <20190722133054.21781-1-mreitz@redhat.com> <20190722133054.21781-3-mreitz@redhat.com> <20190723085236.GA5296@localhost.localdomain> From: Max Reitz Openpgp: preference=signencrypt Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: <91179292-29aa-4d00-78ac-a8861fd5f308@redhat.com> Date: Tue, 23 Jul 2019 11:41:02 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20190723085236.GA5296@localhost.localdomain> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="BnBSMALvW9r0LYlFxSwDDOztUX0t09RR7" X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.43]); Tue, 23 Jul 2019 09:41:06 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH for-4.1 2/2] block: Only the main loop can change AioContexts X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --BnBSMALvW9r0LYlFxSwDDOztUX0t09RR7 Content-Type: multipart/mixed; boundary="YWea4kFR5mnMn4SVO2s9Wmdcy2MgDnQFM"; protected-headers="v1" From: Max Reitz To: Kevin Wolf Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org Message-ID: <91179292-29aa-4d00-78ac-a8861fd5f308@redhat.com> Subject: Re: [PATCH for-4.1 2/2] block: Only the main loop can change AioContexts References: <20190722133054.21781-1-mreitz@redhat.com> <20190722133054.21781-3-mreitz@redhat.com> <20190723085236.GA5296@localhost.localdomain> In-Reply-To: <20190723085236.GA5296@localhost.localdomain> --YWea4kFR5mnMn4SVO2s9Wmdcy2MgDnQFM Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 23.07.19 10:52, Kevin Wolf wrote: > Am 22.07.2019 um 15:30 hat Max Reitz geschrieben: >> bdrv_set_aio_context_ignore() can only work in the main loop: >> bdrv_drained_begin() only works in the main loop and the node's (old) >> AioContext; and bdrv_drained_end() really only works in the main loop >> and the node's (new) AioContext (contrary to its current comment, whic= h >> is just wrong). >> >> Consequentially, bdrv_set_aio_context_ignore() must be called from the= >> main loop. Luckily, assuming that we can make block graph changes onl= y >> from the main loop as well, all its callers do that already. >> >> Note that changing a node's context in a sense is an operation that >> changes the block graph, so it actually makes sense to require this >> function to be called from the main loop. >> >> Also, fix bdrv_drained_end()'s description. You can only use it from >> the main loop or the node's AioContext, and in the latter case, the >> whole subtree must be in the same context. >> >> Fixes: e037c09c78520cbdb6da7cfc6ad0256d5870b814 >> Signed-off-by: Max Reitz >> --- >> include/block/block.h | 8 +++----- >> block.c | 13 ++++++++----- >> 2 files changed, 11 insertions(+), 10 deletions(-) >> >> diff --git a/include/block/block.h b/include/block/block.h >> index 60f00479e0..50a07c1c33 100644 >> --- a/include/block/block.h >> +++ b/include/block/block.h >> @@ -667,11 +667,9 @@ void bdrv_subtree_drained_begin(BlockDriverState = *bs); >> * >> * This polls @bs's AioContext until all scheduled sub-drained_ends >> * have settled. On one hand, that may result in graph changes. On >> - * the other, this requires that all involved nodes (@bs and all of >> - * its parents) are in the same AioContext, and that the caller has >> - * acquired it. >> - * If there are any nodes that are in different contexts from @bs, >> - * these contexts must not be acquired. >> + * the other, this requires that the caller either runs in the main >> + * loop; or that all involved nodes (@bs and all of its parents) are >> + * in the caller's AioContext. >> */ >> void bdrv_drained_end(BlockDriverState *bs); >=20 > I think you are right about the requirement that bdrv_drained_end() is > only called from the main or the BDS AioContext, which is a requirement= > that directly comes from AIO_WAIT_WHILE(). >=20 > However, I don't see why we have requirements on the AioContext of the > parent nodes (or any other nodes), except possibly not holding their > lock. We don't poll their context, so it shouldn't matter in which > context they are? Hm. I don=E2=80=99t know how I got confused there, you=E2=80=99re right.= I don=E2=80=99t think the (falsely given) restriction hurts, though, beca= use a subtree should be within a single context anyway (unless you=E2=80=99re i= n bdrv_set_aio_context_ignore(), which needs to be in the main context). So, hm, yes, I messed up this comment a bit now. But now it=E2=80=99s ju= st more restrictive than it needs to be and I don=E2=80=99t think callers are goi= ng to care, so... Max --YWea4kFR5mnMn4SVO2s9Wmdcy2MgDnQFM-- --BnBSMALvW9r0LYlFxSwDDOztUX0t09RR7 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl021i4ACgkQ9AfbAGHV z0Dmbgf/eMlcH4ArLrFwVQUdP8wukNCHiBrPJEjUVAQmnysca8TGn5xY4OtZ0gcS NJvZhU7fEvXekvJIKAPHV0ZMJH5xSNmES4g6Z66M9i1/tdwcS6nf62RCZJVrDPXW S4ljNwpaCArjVO7JHv8NcMtTtDoNpnicL6rZCj+1u/0F5dNfV2dkR/szwMypYeaE 2BspnO5fnYy2qkTqWuG+FVWUNhPGncuTavoB31b3ZaGThYsdyw+rH6QkgOpoJuMV g4oVLsW/3FSpazz3HynzuGyeAa9PiXyBZV3s+QqriM7uLDjlKbL/4GeYaYA9PkPz BXsNfrXMHSFLlc6SKvUw9fjuvxpxSw== =cQTH -----END PGP SIGNATURE----- --BnBSMALvW9r0LYlFxSwDDOztUX0t09RR7--